Biodegradation of 75 and 100 mg/l of 2,4-dinitrophenol (DNP) by activated sludge acclimated in a Sequencing Batch Reactor (SBR) consistently required less than 6 hours although a lag at the beginning of every 48-hour SBR cycle was observed. Other investigators have reported that DNP levels of 100 mg/l and higher are significantly toxic even to acclimated bacteria. The activated sludge acclimated to 75 mg/l initial DNP had over 100 times the DNP-degrading bacteria than an SBR acclimated to 10 mg/l DNP, although the MLSS concentration in both reactors was similar. Results suggest that two mechanisms are responsible for activated sludge acclimation to toxic levels of DNP: maintenance of DNP-degrading biomass sufficiently large to reduce initial DNP to non-toxic levels, allowing for subsequent rapid degradation; and extension of the aeration period well beyond the time required for degradation to prevent gradual accumulation of any by-product which might also be toxic.